Influence of precursor composition and thermal annealing on structure, phase, purity, and functionality of spray-flame made FexOy

Despite spray-flame synthesis (SFS) enabling versatile production of functional iron oxide nanomaterials, systematic studies quantifying structure-purity-function relationships remain scarce. Moreover, γ-Fe 2 O 3 /Fe 3 O 4 differentiation is accompanied by methodological uncertainties, while methods to systematically compare material purities are lacking. Here, we present a systematic comparison of iron oxides synthesized from iron(III) nitrate nonahydrate (INN) in pure ethanol (EtOH) vs. ethanol/2-ethylhexanoic acid (EtOH + EHA) using the standardized SpraySyn1 burner, employing Mössbauer spectroscopy for unambiguous phase identification. We further introduce particle surface loading ( PSL ) as a size-independent metric for surface impurities, enabling systematic purity comparison across synthesis conditions. Evaluating structure, purity, and functionality measured by vibrational scanning magnetometry (VSM) and oxygen evolution reaction (OER) demonstrates that INN in EtOH + EHA predominantly yields small γ-Fe 2 O 3 particles (